Did this the other day with a friend who also had the exotic bounty. We tried a couple of methods, but in the end the best seemed to be cleaning up adds and ignore the big guy. It took probably 5 or more waves, but eventually a ship came in with Xylor and a whack of baddies at which point we legged it to the starting room. The shrieker that spawns if you fight from there was nothing compared to what was outside so we just kept reviving and whittling away until Xylor came to the front of the pack and it was open season. He is about the same to kill as a regular wizard so not too bad. It helped a lot to use an arc sniper and solar machine gun on the knights and wizards respectively. That was our experiece, good luck guardians!